There are many applications for calendered rubber. One example of an application is roofing material wherein the sheets are rolled out over the surface of the roof to provide waterproofing and insulation. Other illustrative applications are floor matting, conveyor belts, dye cut parts such as gaskets, and protective clothing such as aprons. The preferred thickness of the calendered rubber will generally vary according to the application; typically, the thickness will be in the range from several mils to several thousand mils. Furthermore, a particular rubber compound from the broad categories of natural and synthetic rubbers may have advantage for certain applications.
As is well known, rubber sheeting is commonly formed using a calender wherein uncured or green rubber is pressed between two rotating parallel rollers to form a sheet. The thickness of the sheet is, of course, determined by the separation of the calender rollers. The width and length are also conventionally controlled. Also, the sheet surface characteristics or pattern can be determined by the surface of the rollers.
After passing through the calender, the rubber sheets must be cured to form the requisite molecular bonds which provide the strength and elasticity of rubber. In the prior art, the curing heat has generally been provided by either steam or hot air. Illustrative examples will be described for both of these curing methods.
Super heated steam under pressure is typically applied to the rubber in an autoclave. First, the calendered material is rolled onto a steel drum with a liner separating the layers of rubber to prevent interaction and bonding between the rubber layers. Common materials used for liner separation are cotton, polyester, powder such as talc, and paper. The amount of calendered material rolled onto the drum is greatly limited by the fact that rubber is a very good thermal insulator, and, as such, a large cross-section of material wrapped on the drum would lead to excessive cure time as well as the possibility of scorch or over cure. Generally, the cross-section of material placed on the drum is two inches, limiting the heat transfer requirements to one inch from the drum wall and outside surface.
Drum sizes are usually in the 18 to 24-inch range and are mainly constrained by the autoclave and/or hot air physical size for effective energy operation and capital cost. Generally, the larger the diameter of the drum the greater the amount of rubber that can be effectively cured. In one example, a 22 mil NR-white rubber calendered sheet is rolled on a 24-inch diamter drum to a roll thickness of 2 inches which is approximately equal to 200 pounds of rubber. The drum is then lowered into a pit autoclave and, after the lid is locked, the curing time is 186 minutes. Accordingly, this curing method is time consuming. Even when the throughput is increased by using an autoclave large enough so that six of the above-described roll drums can be processed simultaneously, the throughput is still only about 3100 pounds of rubber per 8-hour shift. Furthermore, energy cost for the steam autoclave has been calculated to be approximately $0.00112 per pound of rubber even when six rolls are processed simultaneously. This calculation is based on an energy cost of $0.05 per kilowatt hour.
The hot air curing method is typically performed in an industrial hot air oven. As with steam curing, the calendered rubber is rolled onto steel drums with liner material separating the rubber layers. In one example, a 57 mil EPDM rubber compound roofing sheet having a width of 54 inches is calendered to a cross-section of 2 inches on a 22-inch diameter steel drum using a 20 mil liner. During curing, the rubber expands so that the cured sheet will have a thickness of approximately 60 mils after processing. For this example, the rubber is cured in stages within a hot air oven. First, it is heated in the hot air oven for three hours at 250.degree. F. Then, the temperature is raised to 290.degree. F. for five hours. Finally, the temperature is lowered to 250.degree. F. for three more hours and, after a total of eleven hours in the oven, the rolls are removed from the oven and cooled for eight hours before being stripped. Even though a plurality of rolls can be cured simultaneously in the same oven if it is large enough, it is still desirable to have substantially higher throughput than is possible with this hot air method. Increasing the driving force by raising the temperature is not a feasible approach to increasing throughput because of the surface temperature sensitivity of rubber compounds. When one or more rolls are cured in the oven or autoclave, they are generally mounted on a fixture or truck. Accordingly, not only do the metal drum and the oven walls act as heat sinks and have to be brought up to temperature, but also, all transport fixtures have to be heated as well. Additionally, the liner material utilized in separating the rubber plies usually has as poor heat transfer characteristics as the rubber thus increasing the time and energy required to reach cure temperature.
In summary, curing of rubber rolled sheets by inward conduction of heat from steam or hot air is very inefficient in terms of time and energy.